andrew s

This effect has been proposed as a solution to the Hubble tension between "local" and CMB derived value. Any very large scale significant non uniformity in the mass energy distribution will impact the local metrical expansion. Regards Andrew

Seems fair enough to me. You can tell if your moving with respect to the CMB by the red / blue shift dipole caused by the motion. It is removed from the CMB images normally shown. Regards Andrew

Yes many times but as @vlaiv said they are not comparable. This is true of both the proposed initial "inflation" and the early stages of the currently observed Hubble expansion. Even now if you go far enough away it is expanding faster than c. Remarkably we can see things that are receding at about 3c. 😊 Regards Andrew

Diffractive optics is already alive and well in commercial photographic lenes https://www.canon.co.uk/pro/infobank/lenses-multi-layer-diffractive-optical-element/ Regards Andrew

Thanks to all. To clarify what I ment by not being mainstream was that I was neither your classic visual observer nor a taker of classic astronomical images. More generally thanks for all the good wishes and I am sure you have not heard the last from me on the forum. Regards Andrew

Today my equipment at Castillejar will be dismantled and put into storage. After nearly 4 yrs of great service from PixelSkies I decided to give up serious observing. Unfortunately, the low resolution spectroscopy I did was fascinating but I could not extract the data I needed. Switching to Exoplanets was productive but just too routine and predictable for my taste. I still retain an interest in astronomy, cosmology and physics but I will also be developing my interest in wildlife and birds in particular. Lots of new spotting scopes, binoculars and photographic equipment to play with. Given I was never a main stream SGLer I will still stick around if that's ok. Regards Andrew

To exclude things from a search put a - sign in front if it e.g. -m31 -galaxy. Make sure there is a space before the - sign. Regards Andrew

Our Universe is not symmetrical! Due to it expansion the Universe is not time symmetric and so energy is not conserved. So no we don't need symmetry for the Universe to work. There are also symmetry breaking mechanisms at the microscopic level as well. However, in everyday life it is locally symmetric so maybe some comfort remains. 😊 Regards Andrew

Emmy Noether should be much more widely known and recognised. Up there with Newton and Einstein in explaining why the world is as we find it. What could be more fundamental than explaining why we see conservation laws and when they are violated. Modern physics rest fundamentally on her work. Regards Andrew

Not all symmetries of interest are in 3d space or 4d space time some are in more complex spaces like Hilbert spaces in QM. Regards Andrew

Both, the same thing can have more than one lable. You can be both a father and son. Regards Andrew

Yes your right just a 180 deg (pi) rotation brings it back to the same state . Regards Andrew

I don't think so as it's a spin 1 particle and 2pi rotation does it. Regards Andrew

It's an example of a modulo sequence. Another example is the 0 to 360 degrees in a circle ( or 0 to 2pi radians). 5 mod(ulo) 4 is 1 etc. It crops up a lot in symmetries in physics. There is even one particle property that needs a 4pi rotation to get to its starting point (twice round the circle) ! Sad to say I can't recall which. Regards Andrew It's spin 1/2 particles. They have a phase shift of -1 for 360 deg rotation and need another loop round to get back to 1

We are all different. One of my delights, as I grow old, is to see the development of my 4 grandchildren. You could not make up such 4 different characters - brilliant. My motivation is/was to understand how things work. Once I am satisfied I know I lose interest. Regards Andrew

Indeed, I think it is the motivation that differs. In fact physicist have led mathematician in some areas. The Dirac delta function was invented by physicists and mathematicians played catch up finally formalising it after years of turning their noses up at it. Regards Andrew

Absolutely, it, as the early posts pointed out, displaces the focal plane. Regards Andrew

Just consider where the principle plane is in a compound telescope. It is not always one of the lenes or mirrors! This illustrates how effective focal length of the whole system is defined using marginal rays. Regards Andrew

Half in jest we used to say Physics is just maths with boundary conditions. If you just give the "variables" fancy names e.g. mass, position, field strength, then it is Physics! Regards Andrew

To be pedantic a plane parallel glass element doesn't change the focal length it shifts the focal plane. Regards Andrew

Mirrorless cameras have built in lens correction including vignetting so it may be due to that. Not sure if it's done in all formats or if you can turn it off. Regards Andrew

Prism diagonals can't be CA free. Try without it. Regards Andrew

Yes it's a complex issue and depends on many variables and as with all things a balance is required. Which noise component dominates is an interesting question. Many of the "standard" answers were derived in the early CCD days and don't reflect the reality of modern CMOS detectors with their small pixels. They suffer significant telegraph noise which is not included in the standard noise equation used by, for example, astroimagej. I have never even seen this discussed recently other than on C Buils spectroscopy web site. It is, however mentioned in the pioneering work of Jansick but was a minor issue compared to others at the time. Another area is scintillation noise in short exposures. Regards Andrew

Astroimagej is very good and I use it for exoplanet photometry. Not the simplest to get into but very easy to use once set up. Regards Andrew

This is not always best practice. If you have bright stars they are often defocused to avoid saturation rather than shortening the exposure. In addition for optimally exposed stars fixed pattern noise dominates and can be exacerbated by switching between a few pixels if guiding is not perfect. It can be reduced by having the star cover more pixels to average it out. Sky background noise can be reduced by by using a good sized annulus. Regards Andrew